For producing optically-active 2-piperazinecarboxylic acid derivatives, for example, known are 1) a method of optical resolution using L-pyroglutamic acid as the resolving reagent (WO 95-21162), 2) a method of optical resolution using an optically-active .alpha.-hydroxy acid such as L-malic acid or the like as the resolving reagent (WO 95-29170), 3) a method of optical resolution using N-tosyl-L-phenylalanine as the resolving reagent (EP 710652), etc. However, as their solubility in water is high, the resolving reagents to be used in 1) and 2) are defective in that their recovery after use is low. On the other hand, as its solubility in acidic aqueous solutions is low, N-tolyl-L-phenylalanine to be used in 3) has the advantage of high recovery after use, but has the disadvantage of low production efficiency in that its ability to dissolve diastereomer salts is low and therefore the salt concentration in the resolving step using it could not be high, and that it requires expensive phenylalanine.
The present invention is to provide a method for producing optically-active 2-piperazinecarboxylic acid derivatives through diastereomer salt resolution, in which the recovery of the resolving reagent used is high and the production efficiency to give the products is high.